PC member Piotr Indyk tells me there were 288 papers submitted to STOC this year, so the acceptance rate was 27.1% (or as Piotr actually put it, 27.083333333333333333%). This is the largest number of submissions in STOC history, but just barely—there were 287 submissions in 2002. This is also the second-lowest acceptance rate in STOC history, at least as far back as I have stats. The theory community's early-00s growth spurt seems to be holding strong. Here's the chart (also in pdf format):